RAF jets could not be scrambled in time to prevent terrorists from crashing a hijacked airliner into the Sellafield nuclear complex, according to an expert analysis passed to the Sunday Herald.

Transatlantic passenger planes forced to divert from their normal flight paths across northwest England would take between four and six minutes to reach the Cumbrian plant. This would hardly give time for the nearest fighters stationed at RAF Leuchars in Fife and RAF Leeming in Yorkshire to leave the ground.

In the wake of 9/11, terrorists flying a fully fuelled jumbo jet into the radioactive waste and plutonium stores at Sellafield is the nightmare scenario. Some estimates have suggested that this could release enough radioactivity to cause long-term cancers among millions of people.

The risk is highlighted by a report on nuclear terrorism due to be published by the UK Parliamentary Office of Science and Technology later this month. Included in the report is an authoritative but unpublished analysis of planes flying close to Sellafield.

The analysis shows that more than 700 airliners pass within 50 nautical miles (93km) of Sellafield every week on their way from Europe to North America. Since they are near the start of their journey, they contain large amounts of fuel which would burst into flames on impact.

A third of the planes are Boeing 747s, the largest civilian airliner in service, carrying an average of 158 tonnes of fuel by the time they near Sellafield. More than 40% are twin-engined Boeing 777s with 124 tonnes of fuel, or Boeing 767s with 63 tonnes.

They follow known Air Traffic Service routes to the east and southwest of Sellafield, often going via the busy Dean Cross radio beacon to the north. They come from 17 different airports, including London, Paris, Kuwait and Tel Aviv.

If any are hijacked and forced to divert to Sellafield from the nearest point, their flight time to the plant is estimated to be between 4 and 5.7 minutes. In times of high alert, RAF fighters at Leuchars and Leeming are on five- minute standby.

Any RAF Tornadoes that were airborne in the nearby Lake District low-flying training area might be able to reach a hijacked airliner sooner. But even if they managed to intercept it, they wouldn’t be able to shoot it down because they don’t carry live weapons on training missions.

“The feasibility of interception therefore appears low if hijacked aircraft are able to take the shortest distances to the target from their planned routes,” the expert analysis concludes.

It would also depend, the analysis observes, “on a political decision to permit the deliberate killing of hundreds of innocent civilians by shooting the aircraft down as a means of preventing it being flown into a nuclear facility”.

The analysis was commissioned after 9/11 from an independent expert by the environmental group Greenpeace. The expert asked to remain anonymous for professional and security reasons.

Although the analysis was based on timetabled flights in October 2001, UK National Air Traffic Services (NATS) has confirmed that the picture is much the same today.

Greenpeace submitted the aviation analysis to the inquiry into nuclear terrorism held by the Parliamentary Office of Science and Technology, which will publish the results of its inquiry in a few weeks’ time.

“If a heavily fuelled jet crashed into Sellafield and hit a target such as the high-level waste tanks, the consequences would be horrific,” said Jean McSorley, the nuclear campaign co-ordinator for Greenpeace UK.

“There would be no going back to normal after such an accident. God knows how people on site or the emergency services would cope.”

The number of fuel-laden flights coming close to Sellafield was “extremely disturbing”, she argued. “We don’t expect the government to disclose exact details of what measures it has taken to move flight paths due to terrorist concerns, but we do have a right to know if this has been dealt with.”

President George W Bush’s special commission on 9/11 revealed last month that al-Qaeda’s original plan had been to hijack 10 planes to attack targets on the west and east coasts of the US. Some of the planes were to be crashed into “unidentified nuclear power plants”.

The UK government’s secretive Office for Civil Nuclear Security said it had been working on “additional measures to counter the risk of a large aircraft being deliberately crashed on to a civil nuclear site”. But its annual report, put on to the Department of Trade and Industry’s website on Friday, added: “Details may not be disclosed for security reasons”.

It nevertheless mentioned that two “substantial” new concrete barriers were being built around the edge of two facilities at Sellafield. “Other measures have also been taken, including strengthened warning procedures and interdiction by RAF aircraft,” the report said.

The Civil Aviation Authority referred inquiries to the Department for Transport, which in turn deferred to the Department of Trade and Industry (DTI). It stressed that security measures at nuclear plants were regulated by the Office for Civil Nuclear Security.

“Security precautions at nuclear sites are kept under regular review and carry a high priority at all times. Following September 11, security enhancements have been put in place,” said a DTI spokesman. “It is not government policy to disclose details of security measures.”